This invention relates to an infusion apparatus and method. It relates more especially to an improved infusion method rechargeable implantable pump for conducting infusate to a selected site in a living body.
Relatively recently, an infusion apparatus has been developed which can be implanted in the body to remain there for a prolonged period. The apparatus can be refilled with infusate without having to remove the apparatus. Refilling is achieved simply by injecting additional infusate through a penetrable septum in the apparatus, which septum is located directly under the patient's skin. In some cases, the act of refilling the apparatus with infusate also recharges the apparatus' power source so that the device can operate uninterruptedly to dispense infusate such as insulin or heparin at a very small flow rate for a prolonged period. An example of infusion apparatus of this general type is disclosed in U.S. Pat. No. 3,731,681.
It has also been proposed to control the infusate flow from such apparatus by means of a valve controlled by a programmable controller so that the dosage rate can be varied to suit particular needs of the patient. For example, in the case of insulin, ideally the patient should receive a small continuous or basal dose to satisfy a particular steady state requirement that depends upon the average amount of sugar in his blood. Then immediately after a meal when blood sugar level rises dramatically, the infusion apparatus should dispense a much larger dose (prolonged bolus) of insulin to offset the increased sugar level caused by the ingestion of the meal. One such programmable infusion apparatus, described in U.S. Pat. No. 4,077,405, varies the valve control pulses in frequency and/or duration to control the duty cycle of an electrically operated valve to provide the requisite basal and bolus doses at the requisite times.
Problems with prior infusion apparatus of the type described above arises because the basal and bolus doses have the same concentration since they both originate from the same infusate reservoir inside the apparatus and incorporation of the necessary batteries, valves and electronic system may reduce system reliability. With the bolus insulin rate requirement being ten to fifteen times that of the basal rate, it will become obvious that the mean insulin requirement will quickly consume the reservoir volume if the concentration is set to meet the requirement of the basal condition. The desirability of the system is further reduced due to the need for the batteries for the electrical power source. These must be periodically recharged or replaced. Furthermore, the infusate reservoir has to be quite large to provide both basal and bolus flows while at the same time avoiding the need for frequent refilling. The large reservoir of course increases the overall size and weight of the pump package implanted in the body to the discomfort of the patient.
Prior controllable infusion apparatus is disadvantaged also in that it is possible for the patient to inadvertently or intentionally control the apparatus so as to administer an overdose of infusate. This not only exhausts the supply of infusate but also can result in injury to the patient. An insulin overdose, for example, can bring on hypoglycemic shock resulting in death to the patient.
More generally, however, it would be desirable to provide implantable infusion apparatus which is small and compact so as to occupy a minimum amount of space in the body, yet which is able to supply the patient's infusate needs while still avoiding the need for frequent refills or consumable power sources such as batteries.